Differential radius transmission line length adjuster

ABSTRACT

Means are disclosed for very accurately adjusting the length of a strip transmission line wherein the transmission line has two open radial ends of different radii disposed adjacent each other and a rotatable transmission line having two radial portions which overly the radial open ends to complete the circuit. The radial portions are joined by a continuously smooth transitional section, and the change in length is inversely proportional to difference in radii of the two radial portions.

United States Patent [191 [111 3,742,400 Friend June 26, 1973 DIFFERENTIAL RADIUS TRANSMISSION LINE LENGTH ADJUSTER OTHER PUBLlCATIONS Strip Transmission Line Stretcher by Forbes RCA Technical Notes, RCA-TN-No. 338, Nov. 59

Primary ExaminerRudolph V. Rolinec Assistant ExaminerSaxfield Chatmon, Jr. Attorney-Mueller and Aichele [57] ABSTRACT Means are disclosed for very accurately adjusting the length of a strip transmission line wherein the transmission line has two open radial ends of different radii disposed adjacent each other and a rotatable transmission line having two radial portions which overly the radial open ends to complete the circuit. The radial portions are joined by a continuously smooth transitional section, and the change in length is inversely proportional to difference in radii of the two radial portions.

8 Claims, 4 Drawing Figures PAIENTEU JUN 2 8 I973 SMEIIUFZ INVENTOR Lawrence 0 Fr/md WM aw PAIENIEmunas ma 3. 742.400

sum 2 or 2 set X 020 RKS 6 32\33 IO 45A V TO ADJUST INVENTOR Lawrence 0 Friend arrrs.

DIFFERENTIAL RADIUS TRANSMISSION LINE LENGTH ADJUSTER BACKGROUND OF THE INVENTION This invention relates to means for changing or adjusting the length of a strip transmission line for microwave frequencies, more particularly to means for changing or adjusting the length of a stripline section and it is an object of the invention to provide improved means of this character.

In many microwave applications, in the gigahertz range, for example, phased arrays, balanced mixers, sum and difference networks, stub tuners, microwave filters, directional couplers, etc., it is essential that the strip transmission line or lines be of a precise length. One form of such a transmission line is termed stripline and comprises a sandwich of two metallic ground planes, two layers of dielectric material and thin strip of conducting material between the layers of dielectric material. In some instances wherein two or more transmission lines, such as striplines, are utilized, it is essential to the proper operation of the device that the lengths of the transmission lines be equal to each other within a few thousandths of an inch. Such precision is difficult, if not impossible, to obtain by the usual printed circuit, photolithographic techniques or other techniques which involve, in the first instance, mechanically determining the length of the transmission line.

Adjustable tuners, phase shifters, etc., for the microwave region known to the prior art have lacked the necessary precision, and have been bulky, difficult to make and expensive.

Accordingly, it is a further object of the invention to provide improved means for mechanically adjusting the length of a stripline with great accuracy.

It is a further object of the invention to provide improved apparatus of thecharacter indicated which is simple in form, inexpensive to make and efficient in operation.

SUMMARY OF THE INVENTION In carrying out the invention in one form, means for varying the length of a strip transmission line are provided comprising a strip transmission line portion having two open ends extending from opposite directions and terminating adjacent each other in concentric radially spaced sections, and a concentrically rotatably movable strip contacting each of said concentric radially spaced sections for differentially changing the lengths of said radially spaced sections in said strip transmission line portion.

The change in length of the strip transmission line is governed by the low F (r m-r wherein F is the change in length factor, r,, is the radius of the rotatable index, r, is the radius of one end section and r is the radius of the other end section.

BRIEF DESCRIPTION OF THE DRAWINGS on aneenlarged scale of the apparatus shown in FIG. 1;

FIG. 3 is a sectional view on a larger scale taken substantially in the direction of the arrows 3-3 of FIG. 2, and i FIG. 4 is a figure similar to FIG. 3 taken substantially in the direction of the arrows 4-4 of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, the invention is shown in a stripline circuit forming part of a balanced mixer and comprising a ground plane 10, a layer of dielectric material 11 including a metallic strip transmission line 12 on the top surface thereof, a thin layer of electrical insulating material 13, a second layer of dielectric material 14 including a metallic strip transmission line 15 on the underside thereof, and a second metallic ground plane 16.

The ground planes 10 and 16 while shown as being of relatively bulky material may be substantially thinner if desired and are utilized in the forms shown for reasons of simplicity and in order to provide sufficient metal for the attachment of connectors or terminals such as connectors l7, l8 and 19 to the stripline circuit. Terminals 1'7, 18 and 19 may comprise terminals for coaxial cables, for example, and may be attached to the ground planes 10 and 16 by screws 21.

Machined out of the upper surface 22 of the ground plane member 10 is a depression 23 which is of the same shape as, and the combined thickness of, the dielectric layers 11 and 14 so that when these layers are laid upon each other and disposed in the depression 23, the upper surface 24 of the dielectric layer 14 is essentially flush with the top surface 22 of the ground plane 16. The ground planes 10 and 16 are held to each other and tightly hold the dielectric layers 11 and 14 within the depression 23 by suitable screws (not shown) passing through holes 25 in ground plane member 16 and being threaded into threaded openings 26 in ground plane member 10.

The transmission line 12 formed on the top surface of dielectric layer 11 begins at a straight portion 27 disposed adjacent the terminal 18 and terminates in a portion, or end, 28 which cooperates with terminal 19. In between the portions 27 and 28, the transmission line 12 has a length and shape determined by the particular purposes for which the circuit is intended. The transmission line 15 formed on the undersurface of dielectric layer 14 includes an end portion 29 which in the assembled structure overlies the end portion 27 of transmission line '12 and cooperates with the terminal 18. The other end of transmission line strip 15 designated by the reference character 31 is adapted to cooperate with the terminal 17. The transmission lines, or strips,

' l2 and 15 have the same shape, but are mirror images of each other, in effect. It is intended that the transmission line strips 12 and 15 have the same length very precisely, for example, within a few thousandths of an inch inbetween the beginnings 27 and 29, and the ends 28 and 31. It is to the achievement of the latter feature that the subject invention is principally directed.

The adjustment of the transmission line strip 15 is achieved by means of the rotating member 32 on which is mounted a portion of a transmission line strip 33 (part of 15) to be more fully described and which coopcrates with the end sections 34 and 35. The end section 34 terminates the portion 36 of the transmission line strip 15 and the end portion 35 terminates the portion 37 of the transmission line strip 15.

In the assembled condition, the dielectric layer 11 including the rotatable member 32 is first disposed in the depression 23 of ground plane 10. The insulating film 13, shown transparent, then overlies the U-shaped portion 28a of transmission line strip 27. The dielectric layer 14 is then disposed on top of the dielectric layer 11, the depression 23 being of sufficient depth that the top surface 24 of the dielectric layer 14 is essentially flush with the surface 22 of the ground plane 10. In this position the rotating member 32 has its central opening 38 disposed over a pin 39, and the projecting portion 41 of the dielectric layer 14 is disposed over top of the rotating member 32, an opening 43 being provided and is received over the pin 39.

Over top of the dielectric layer 14, the ground plane 16 is disposed, and when the length of the transmission line has been adjusted, as will be described, screws 25 are applied for holding the assembly together.

The insulating film 13 which, for example, may be made of Teflon, in overlying the U-shaped portion 280 of the transmission line strip 12, insulates the transmission line strip 28a from the portion 36 of the transmission line 15.

The rotatable adjusting member 32 has a tab 43 thereon by means of which member 32 may be rotated and an index mark 44 is disposed thereon for cooperation with an index scale 45 formed on the projecting portion 41 of the dielectric layer 14. The radius of the index scale 45 is shown as being the same as that of the rotating member 32 as will become clear, and the distance between the marks on index scale 45 is such that they may be easily read by the human eye, for example, two-hundredths of an inch apart.

Referring to FIG. 2, the structure illustrated may be considered as an enlarged plan view of the projecting portion 41 in the assembled form of the apparatus, but with the top ground plane 16 removed and a portion of the dielectric layer 14 broken away as shown by the line 46 to expose a portion of the rotating member 32 and the strip transmission line portion 33 thereon.

The transmission line portion 33 consists of two portions, one portion 47 having a radius r,, and the other portion 48 having a radius r The transition between portions 47 and 48 is smooth and without discontinuities, a curved transitional area being shown as typical. The transmission line portion 47 extends to the dotted line 49 and the transmission line portion 48 extends to the dotted line 51. The circular end section 34 of the portion 36 of transmission line strip 15 has the same radius r as the transmission line portion 47. Similarly, the circular end section 35 of the portion 37 of transmission line strip 15 has a radius r,, the same as the transmission line portion 48. Thus it will be readily apparent when considering FIGS. 2, 3 and 4 that the transmission line section 34 overlies and contacts the transmission line portion 47, and the transmission line section 35 overlies and contacts the transmission line portion 48. The circular section 34 terminates at the dotted line 52 and the circular section 35 of the transmission line 15 terminates at the dotted line 53.

The length of the transmission line strip 15 between the ends 29 and 31 includes the circular portions 34 and 35 between their respective tennini 52 and 53 and the circular portions 47 and 48 of the transmission line strip 33 on the rotatable member 32 which extend between the same termini 52 and 53. The latter consist of those circular portions 47 and 48, and the transitional section 54 between them. It will be evident that the length of the transmission line strip between the termini 52 and 53, consisting of portions 47 and 48 having different radii, will be longer or shorter depending upon the relative proportions of the circular portions 47 and 48 are functioning at a particular setting. Thus if the tab 43 is about in its center position whereby the index mark 44 is midway of the extremes of its potential movement, there may be about equal proportions of circular portions 47 and 48 forming the connecting transmission line strip between termini 52 and 53. On the other hand, if the tab 43 is moved to its right-hand extreme as shown by the dotted line 55, there will be a lesser proportion of the transmission line portion 48 and a greater proportion of the transmission line portion 47 between the termini 52 and 53 thereby giving the transmission line a shorter length because the radius r is less than the radius r Similarly, if the tab 43 is moved to its extreme left-hand position as shown by the dotted line 56, a lesser proportion of transmission line strip 47 is utilized and a greater proportion of transmission line strip 48 is utilized thereby lengthening the transmission line strip 15 because the radius r is greater than the radius r,.

The index scale 45 described earlier in this application has marks 45a, for example, thereon for cooperation with the index mark 44 for accurately adjusting the position of the tab 43. The index scale 45 has a radius r;, which is larger than either of the radii r and r and the arcuate or circular spacing between any two index marks 45a may, for example, be two-hundredths of an inch because this spacing can be readily read by the human eye. Other spacings to fit particular positions may of course be utilized.

The change in length of the transmission line between the termini 52 and 53 is determined by the ratio of (r m-r For example, if the radius r were 1.75 inches, the radius r were 1.5 inches and the radius r were 1.25 inches, the change in length factor, F, would be equal to (l.75/l.5 1.25), or 7. Then for a movement of the index mark 44 from one mark 45a to another would be equal to 0.020/7 or 0.003 inches. Thus it will be evident that changing the position of the tab 43 by one index mark produces a very small change in the length of the transmission line.

Referring to the expression F (r /r r,), it will be evident that the difference between the two radii of circular portions 47 and 48 is the more significant factor in the relationship which determines the change in the length of the transmission line because the radial difference occurs in the denominator of the factor F. The factor F may be made very large as the difference between the two radii r and r approach each other. It will be less if the difference between the two radii is made larger. The change in length of the transmission line between the two termini is therefore essentially dependent upon the differential radius, and more specifically, is inversely proportional to the differential radius.

At higher microwave frequencies such as 18 gigahertz, for example, it is necessary that the length of the two transmission lines 12 and 15 be the same within one or two thousandths of an inch. By good design procedures which may be of the printed circuit variety wherein photolithographic methods are used to form narrow conducting strips accurately on a dielectric layer, the length of the transmission lines 12 and 15 may be made very nearly the same length in the first instance. For example, in order to achieve this, the transmission line 12 includes an arcuate portion 12a which simulates the arcuate portion of the transmission line formed by the various portions of the arcs on the movable tab 32 and the arcuate sections 34 and 35. It is difficult however in the initial design of the circuit to make the transmission line length precisely the same such, for example, as within one or two thousandths of an inch. By utilizing the invention wherein the difference between the two radii r and r is made very small, appropriate adjustments can be made.

As may be seen in FIG. 2 when the tab 43 is in its extreme right-hand position 54, the arcuate, or circular, portion 48 terminates at the dotted line 57 and when the tab is in its extreme left-hand position, the arcuate or circular portion 47 terminates at the dotted line 58.

The ability to accurately change the length of the transmission line makes possible very accurate phase adjustments at the high microwave frequencies, makes it possible to create accurate phase trimming of stripline microwave circuits and may have equal application to structures such as phased-arrays, balanced mixers, sum and difference mixers, stub tuners, microwave filters and directional couplers.

The dielectric members 11 and 14 may be made of any suitable high frequency dielectric material, such for example, as that available under the name Bexolite, and the thickness thereof may be selected to give the desired properties at the frequencies being used.

What is claimed is:

1. Means for varying the length of a strip transmission line comprising:

a strip transmission line portion having two arcuate open ends whose arcuate portions extend from opposite directions toward each other and terminate adjacent each other in concentric radially spaced sections; and a concentrically rotatably movable strip having two connected radially spaced portions contacting, respectively each of said concentric radially spaced sections for differentially changing the lengths of said radially spaced sections in said strip transmission line portion.

2. The transmission line length varying means according to claim 1 including:

index means having a radius greater than the greater of the radii of said concentric radially spaced sections.

3. The strip transmission line length varying means according to claim 1:

wherein the two radially spaced portions of said concentrically rotatably movable strip have ends which extend arcuately away from each other in opposite directions, one portion for contacting one of the concentric radially spaced sections of said transmission line portions and another portion for contacting the other one of the concentric radially spaced sections of said transmission line portions; and

a continuous, smooth transitional section between the two radial portions of said rotatably movable strip.

4. The transmission line length varying means according to claim 3:

including index means having a radius greater than the greater of the radii of said concentric radially spaced sections.

5. Means for varying the length of a strip transmission line comprising a strip transmission line section having two ends adjacent each other, one of said two ends terminates in an arcuate section having a first radius, the other of said two ends terminates in an arcuate section having a second radius differing from said first radius, the radii of said arcuate sections having the same center and being disposed on opposite sides thereof for directing said arcuate sections extend toward each other from opposite directions, a rotatable transmission line having two arcuate portions disposed in contact with the arcuate sections of said strip transmission line, the contacting arcuate portions of said rotatable transmission line having radii equal respectively to said first radius and to said second radius, said rotatable transmission line being rotatable for changing the length of said strip transmission line in inverse proportion to the dif ference in said first and said second radii.

6. The strip transmission line length varying means according to claim 5 including index means having a radius greater than that of said first radius and said second radius.

7. The strip transmission line length varying means according to claim 3 wherein the strip transmission line comprises a stripline.

8. Means for varying the length of a strip transmission line comprising two transmission line sections having adjacent open radial ends of different radii, and length varying means having different radii between said open ends and in contact therewith, a rotating member supporting said length varying means, said length varying means operating according to the law F (r m-r wherein F is the change in length factor, r, is the radius of one of said ends, r, is the radius of the other one of said ends and r is the radius of the rotating member. i 4 t i 

1. Means for varying the length of a strip transmission line comprising: a strip transmission line portion having two arcuate open ends whose arcuate portions extend from opposite directions toward each other and terminate adjacent each other in concentric radially spaced sections; and a concentrically rotatably movable strip having two connected radially spaced portions contacting, respectively each of said concentric radially spaced sections for differentially changing the lengths of said radially spaced sections in said strip transmission line portion.
 2. The transmission line length varying means according to claim 1 including: index means having a radius greater than the greater of the radii of said concentric radially spaced sections.
 3. The strip transmission line length varying means according to claim 1: wherein the two radially spaced portions of said concentrically rotatably movable strip have ends which extend arcuately away from each other in opposite directions, one portion for contacting one of the concentric radially spaced sections of said transmission line portions and another portion for contacting the other one of the concentric radially spaced sections of said transmission line portions; and a continuous, smooth transitional section between the two radial portions of said rotatably movable strip.
 4. The transmission line length varying means according to claim 3: including index means having a radius greater than the greater of the radii of said concentric radially spaced sections.
 5. Means for varying the length of a strip transmission line comprising a strip transmission line section having two ends adjacent each other, one of said two ends terminates in an arcuate section having a first radius, the other of said two ends terminates in an arcuate section having a second radius differing from said first radius, the radii of said arcuate sections having the same center and being disposed on opposite sides thereof for directing said arcuate sections extend toward each other from opposite directions, a rotatable transmission line having two arcuate portions disposed in contact with the arcuate sections of said strip transmission line, the contacting arcuate portions of said rotatable transmission line having radii equal respectively to said first radius and to said second radius, said rotatable transmission line being rotatable for changing the length of said strip transmission line in inverse proportion to the difference in said first and said second radii.
 6. The strip transmission line length varying means according to claim 5 including index means having a radius greater than that of said first radius and said second radius.
 7. The strip transmission line length varying means according to claim 3 wherein the strip transmission line comprises a stripline.
 8. Means for varying the length of a strip transmission line comprising two transmission line sections having adjacent open radial ends of different radii, and length varying means having different radii between said open ends and in contact therewith, a rotating member supporting said length varying means, said length varying means operating according to the law F (r3/r2-r1) wherein F is the change in length factor, r2 is the radius of one of said ends, R1 is the radius of the other one of said ends and r3 is the radius of the rotating member. 